My co-pending United States patent application Ser. No. 08/629,161, filed on Apr. 8, 1996, now U.S. Pat. No. 5,645,023, discloses a valve train mechanism which utilizes a finger follower provided with a roller which is in continuous engagement with a rotating cam lobe formed on a camshaft. The roller is supported for rotation by a shaft which is secured to an intermediate portion of the finger follower. As is well known to those skilled in the art of designing valve train mechanisms for internal combustion engines, a roller assembly of the type seen in the above-mentioned application has its supporting shaft retained by the finger follower by press-fitting the shaft into the side walls of the finger follower. In addition, in order to provide a low friction rolling motion of the roller, a series of circumferentially spaced needle bearings are interposed between the inner circular opening within the roller and the shaft.
One problem with the above-described roller assembly is that the finger follower must be much wider than needed so as to include side walls of sufficient strength in order to permit the press-fitting of the support shaft to the side walls. As should be apparent, a design of this type increases the weight of the finger follower. Moreover, the heavier finger follower demands a heavier valve spring and creates unnecessary dynamic problems during valve actuation. Such problems could be reduced lift of the valves, or increased forces, or limited engine speed.
Another problem with the roller assemblies disclosed in the above-identified application is that the diameter of the roller shaft is critical in controlling the shaft strength on bending and the shaft may also require a large diameter to prevent wear of the needle bearings. As a result, these considerations tend to control the diameter of the roller and can result in having a shaft diameter much larger than desired. There are two reasons for this. First, the shaft needs a certain radial wall thickness on the side wall bosses of the finger follower for support strength. Since the roller in contact with the cam-lobe must protrude over the maximum diameter of the shaft support bosses to avoid the cam-lobe from contacting the bosses instead of the roller, the roller radius is forced to be 1-1.25 mm larger than the radius of the bosses. This, in some cases, forces a larger than required roller and tends to increase the mass of the rocker arm and the spring forces to the detriment of the valve dynamics. Second, the radial stack-up of necessary dimensions outwardly from the center of the shaft for shaft strength and needle life also may force a larger than required roller diameter.